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What turn rates are expected by ATC when given expedited turn instructions?
One can reasonably expect that for a normal turning instruction (such as a radar vector), ATC will expect you to make a standard rate turn, or perhaps a half rate turn in the high altitude environment.
However, when ATC gives an "expedite turn" on a radar vector or equivalent instruction such as the "Traffic Alert, Turn Left Immediately" used for PRM approach breakouts, what level of performance are they expecting the pilot to demand out of their airplane? A 1.4g (45deg bank) steep turn? A 2g (60deg bank) steep turn? Maximum sustained Gs, even if that means using afterburner in a fast jet?  Get on this heading ASAP even if it means trading a bit of altitude for energy?  Does it depend on whether a light aircraft, a heavy jet, or a fast jet is being vectored around?

The air traffic controller expects you to 1) start your turn immediately and 2) turn as quickly as can safely be done. We do not have all the facts, and do not know exactly what sort of turn you are capable of doing. Using a phrase such as "expedite turn" or "turn immediately" basically means - I've screwed up, you're in danger, watch out and get out of the way. It is then up to you as pilot in command to avoid a collision, by flying your aircraft and knowing its limitations.
When ATC separation has been lost, you should respond promptly to any instructions issued with the purpose of re-establishing separation. We will also, as far as practical, issue traffic information, and expect you to look out the window and take any avoiding action you find appropriate to prevent a collision.
Finally, as a side note, if you have a TCAS, please remember that a TCAS RA takes precedence over any instruction issued by ATC.

Good that you mention the precedence of TCAS over ATC instructions. However TCAS RA,s are in the vertical plane, turns are horizontal. It seems they can be combined as long as the aircraft has sufficient performance to follow the RA in a turn.

@DeltaLime You are quite right. TCAS II RA's provide only vertical resolutions, and so ATC instructions to turn should not conflict with TCAS. TCAS III will probably include horizontal resolution advisories as well, but that is well into the future.

IMMEDIATELY− Used by ATC  or  pilots when such action compliance is required to avoid an imminent situation. - FAA Controller pilot glossary1

The meaning of the word "immediately" is fairly imprecise. It does not specifically lay out within what parameters the pilot is expected to perform an instruction. It does imply a level of urgency, but the controller's interpretation may differ from the pilot's.
The relevant ICAO documents, ICAO Doc 4444 (Chapter 12) and ICAO Doc 9432 use the word “immediately” to imply urgency. ICAO recommends using the word "immediately" with the phrase "due traffic" when instant action is needed from the pilot.2
The term may also differ between different countries.
The UK Manual of Radiotelephony CAP 413. draws a distinction between “now” and “immediately” as follows:

“Use of the word 'now' indicates that the instruction should be complied with in accordance with normal aircraft operating procedures, but without delay. Use of the word 'immediately' indicates a further degree of urgency exists (e.g. to avoid flight into terrain or restricted airspace, or for the provision of collision avoidance, see Chapter 5 Paragraph 1.6.4 Avoiding Action Phraseology). In such circumstances, the pilot should take action to comply with the instruction as soon as practicable, subject to the safety of the aircraft.”3

Another word that can be used, which indicates a higher level of urgency is "expedite." It is usually used in terms of climb and descent, but after giving a turn instruction the controller can use the phrase "expedite turn" to indicate more urgency.

EXPEDITE−  Used  by  ATC  when  prompt  compliance  is required to avoid the development of an imminent  situation. Expedite climb/descent normally  indicates to a pilot that the approximate best rate of  climb/descent should  be  used without requiring an exceptional  change in aircraft handling characteristics.1

A tragic example of the ambiguity of the word "immediate" is the mid-air collision between an F-16 and a Cessna 150 in 2015. When the controller noticed the conflict she gave the F-16 a traffic advisory and when the pilot did not appear to have a visual she gave the order "if you don't have that traffic in sight turn left heading 180 immediately."
Unfortunately the controller's expectation of the word "immediately" did not match the pilot's interpretation.

In postaccident interviews, the controller stated that when she issued the command to the F-16 pilot to turn left "immediately," she expected that the F-16 pilot would perform a high performance maneuver and that she believed that fighter airplanes could "turn on a dime."4

Instead the pilot began a standard-rate turn using the autopilot. This rate ended up being just enough to bring the F-16 directly into the flight path off the Cessna causing a mid-air collision that killed the two people aboard the Cessna. The NTSB report determined that "further clarification of her expectation, such as directing the pilot to "expedite the turn," would have removed any ambiguity."4

1 FAA Pilot Controller Glossary
2 Skybrary Urgency Instructions and Clearances
3 UK Manual of Radiotelephony CAP 413.
4 NTSB probable cause report:'14 CFR Armed Forces
Accident occurred Tuesday, July 07, 2015 in Moncks Corner, SC

Air Traffic Controllers are aware that the higher and faster you are the bigger your turn and factor that into normal vectors.
However when a controller gives a Traffic Alert, Turn Left Immediately the controller expects that the  aircraft will give the best turn they can. That is because the controller said Traffic Alert, the pilot knows it is in their best interest to get on that heading as fast as possible. The immediately is there to say don't start the turn in 30 seconds, start it now, and gives a bit more emphasis on the severity of the situation.

Part of the question though is "does ATC really want a F-16 to pull a 9-G turn when given an expedited vector or traffic alert?"

@UnrecognizedFallingObject A 9-G turn is probably beyond what they're accustomed to seeing, but if you can do one, have at it. An airliner probably won't exceed 30 degrees bank unless the pilot visually sees the traffic & the need to go to more (very rare). The salient point though is, start it ASAP & turn as quickly as you can... too fast/too much turn isn't the concern. Too late and/or too slow is what needs to be avoided.

@Ralph that is exactly the point I am trying to make.

@UnrecognizedFallingObject if they want to pull 9-G turn then at least the issue would be resolved. But I doubt they would because they would probably break a squadron regulation.


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What are the evenly-spaced "bumps" on Bell X-1 tailplane?
The tailplane of the Bell X-1 had four evenly-spaced "bumps" on each side--what was their purpose?  (Screen shot from The Right Stuff.)

Another image

If they're hinges for the elevators, then how come the rudder and ailerons have flush hinges?

They are indeed hinges, and I'm guessing (hence not an answer) that the tail airfoil just wasn't thick enough to handle hinges of the size required for the aerodynamic forces.

@RonBeyer: No, they hold the counter weight of the elevator, and to avoid flutter this counter weight is distributed over span.

@PeterKämpf When they changed to the all-moving stabilizer did the retain the elevators or did they fix them to the stab? Getting confused reading about it. Some people seem to be describing it as a stabilator, but some seem to describe it more like a trimmable stab and elevator

Btw, there's a pic where you can see them closer here http://www.skytamer.com/1.2/2002/2136.jpg

@TomMcW: Trimmable stab and elevator is right. Stabilators were very unusual in the 1940s.

I remember reading in Chuck Yeager's biography that the X-1 could remain stable in supersonic flight because of its all flying tail, something Wikipedia seems to confirm.

@ymb1: Too many reputation points …

This documentary is about the X-1 breaking the sound barrier. It shows that the bumps are indeed hinges, and that the horizontal tail was indeed originally designed as a trimmable stab and elevator. At 25:40 the control surfaces can be seen moving.
At 35:00 Chuck Yeager talks about losing control at M.94 with this configuration, and then having the tail config reworked into an all flying tail, directly connecting the stabiliser to the stick.
Update Aug 2019
The original clip has been removed, this is the new link. Starts at the movement of the all flying tail.

That youtube video has been removed.  Is there another copy online somewhere?

@CamilleGoudeseune looks like this one is similar, time stamps are a.bit later

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What causes Mach No. to increase with altitude at a constant cost index?
If a jet aircraft flies at a constant cost index but changes flight level the Mach no required by the FMC will increase with increasing altitude and decrease with decreasing altitude. Why is this? (Disregard aircraft weight changes due to fuel burn etc)

This sounds an awful lot like a homework assignment.

In other words: does the aircraft need to burn less fuel at higher altitude?

Cost index is a function of Time Cost/Fuel Cost and is more related to True Air Speed than Mach Speed since Mach Speed varies with altitude.
At FL300 M.85 equals 501 knots but at FL400 M.85 is only 487 knots. In order to maintain 501 knots at FL400 you need to increase your Mach speed to M.87

Both of these terms are not related. There is no cause effect relation between them. That is the reason why there is a constant cost index even though the mach number changes as the altitude increases. This is sufficient to answer your question directly. If you are wondering how these data are unrelated, continue reading.
This may get quite technical. Please bear with me.

COST INDEX
As said above, cost index is the ratio of unit cost of time to the unit cost of fuel. This is used to optimize the aircraft's airspeed.
About Airspeed
Aircraft's airspeed is the speed of the aircraft relative to air in it's vicinity and this is different from ground speed (which is defined as the relative speed of the aircraft with respect the ground). Please note that there are 3 types of airspeed (true, indicated, calibrated). For now, just consider the airspeed.
Jet Engine Theory
What jet engines do are quite simple:

Absorb the air
Compressor compresses the air and pushes it backwards
Due to compression, air heats up and pressurizes and wants to expand
The nozzle is convergent and the ambient air pressure is low. So this gives an extra boost to expanding air.
Newton's third law. The air expelled propels the engine forward

Temperature v/s Altitude
Temperature of the air decreases with increasing altitude
Efficiency of the jet engine
The net change in pressure created by the compressor of the jet engine

at ground level, is quite high (Sorry, no numbers. But it is high enough to easily provide enough velocity for lift-off).
at higher altitudes, this is even higher. Why? The lower temperature of air gives room for higher change in air temperature. And thus higher air pressure. And thus, more thrust.

How does this related to cost index?
Well, in order to optimize fuel consumption, it is necessary to maintain near constant airspeed (which is optimized based on individual flight parameters). So, now you may understand, why sometimes you are flying slower on a map and why sometimes faster. This is because, the wind-speeds vary a lot. And airlines try to maintain a constant airspeed (not constant ground speed). Saves a lot of fuel.

Mach Number
\begin{equation}Mach\space Number = Aircraft\space Airspeed / Speed_{\space Sound\space in\space Air}\end{equation}
As explained above, the aircraft's airspeed is maintained as constant as possible coping to changes in external wind speeds. However, what actually makes mach number higher, is the denominator part.
\begin{equation}Speed_{\space Sound\space in\space Air} = \sqrt{\gamma\space R\space T} \end{equation}
Speed of sound in air decreases as the altitude increases. This is due to changes in properties of air with increasing altitude.
And thus, mach number increases even though the airspeed is almost constant.

Hope this helps in discerning how cost index and mach number are independent of each other. Cheers.

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How can an airline flight crew handle an in-flight oxygen system fire?
The first step in the "Smoke, Fire, or Fumes" checklist for any pressurized aircraft is for the crew to put on their oxygen masks and set them to EMERGENCY, so that the crew can breathe, and thus function well enough to execute the rest of the checklist.
However, it's possible on at least some aircraft for a fire to develop that intimately involves the flight crew supplemental oxygen system, as the hull loss of SU-GBP aka EgyptAir flight 667 at its gate in Cairo Int'l demonstrated dramatically.  Fortunately for the EgyptAir crew, their experience happened with the aircraft squarely on terra firma, allowing a swift evacuation with only a few serious injuries.  However, if a similar event happened while cruising over the ocean at 35,000', what could the flight crew do to avoid being incapacitated, considering that such a fire would render at least part of the crew oxygen system inoperable?

Isolate the main cylinders and saturate fire co2/haylon from the fire extinguishers. Oxygen in elevated concentrations will raise the heat and intensity of any fire in quite a spectacular manner.

@jCisco Saturating fires with CO2 in a confined space seems very dangerous. Do they actually use CO2 for this on planes?

Similar event (probably; investigation still ongoing, but the similarity is significant) did happen while cruising over o̶c̶e̶a̶n̶ sea at 3̶5̶,̶0̶0̶0̶'̶
FL370, by coincidence also to Egypt Air, with predictably catastrophic results.

@DavidRicherby I'm not sure if CO2  based suppression systems or extinguishers would be available on a commercial passenger aircraft. Halon would be more suitable and safer primarily mitigating the reactivity, cold shocking risks to the aircraft and suffocation of the crew and passengers.

Opening the flight deck door may be sufficient in some minor cases at the expense of letting smoke and CO2 invading the cabin volume.
In serious cases: Protective breathing equipment is available in the aircraft:

Smoke hood demonstration video:

Portable oxygen cylinders and masks are also available, e.g. for A320 family:

(source)
The presence of this equipment is regulated, e.g. in US by 14 CFR 25.1439:

at least one portable protective breathing equipment shall be located at or near the flight deck for use by a flight crewmember.
In addition, portable
  protective breathing equipment must be installed for the use
  of appropriate crewmembers for fighting fires in compartments
  accessible in flight other than the flight deck. This includes
  isolated compartments and upper and lower lobe galleys, in which
  crewmember occupancy is permitted during flight. Equipment must be
  installed for the maximum number of crewmembers expected to be in the
  area during any operation.

The oxygen supply if for 15 minutes minimum:

The equipment must supply protective oxygen of 15 minutes duration per
  crewmember at a pressure altitude of 8,000 feet with a respiratory
  minute volume of 30 liters per minute BTPD.

These systems are already necessary to the cabin crew to fight any fire or help passengers in case of cabin depressurization.
If a safe aircraft state cannot be restored within the 15 minutes, then the fire is already uncontrollable. Over the ocean and too remote from usable airports, the crew would have to resign and ditch the aircraft to evacuate it, a last resort and risky escape action.
In the accident you mention:

The oxygen system involvement/malfunction is suspected but not certain.
Considering the damages to the flight deck and panels (seen below), a problem for pilots would have been to be able to fly the aircraft with missing or failed instruments and controls, in a hot flight deck atmosphere. The passengers and crews seem to owe their lives to the accident occurring on the ground.

(avherald.com)

While awful to wear the smoke hoods can be brilliant. My father-in-law is a firefighter and sister-in-law an ex air hostess so both have had training in them. They said there are several around the cabin along with portable oxygen supplies, which are used for any serious fire.


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How much FOD hazard can a Styrofoam drink cup pose?
So, the Collings Foundation and their touring warbirds were in town recently, and we went to visit them.  While out on the ramp though, my father managed to lose one of the Styrofoam coffee/drink cups the FBO provides (for their complimentary ice water and iced tea, of course).  How much FOD hazard could that runaway Styrofoam cup pose, considering its relative lack of hardness and stiffness/strength compared to most other FOD hazard items?

Probably very little. There's the very slight chance that it could stuck on a pitot tube or some other terrible scenario in a Final Destination type film which, coupled with another issue, could cause a problem but again very unlikely.
The underlying hazard is the attitude towards FOD in general. It's a bad habit to build complacency towards any FOD sitting on a parking apron or letting it "get away". If it's SAFE to pickup the FOD then it should be picked up. It's possible the next time someone ignores a cup, it could have been used by a wayward mechanic to hold a sticky chemical that would ruin a pilot's windshield and his day if it  smacks and smears on the glass on takeoff. Again very unlikely but why risk it!

It was the "thing that you knew you stuck in your pocket but came out at some point" sort of scenario, BTW...

Chances of it causing a real damage are slim but not at all negligible. Shredded by a propeller may end up in a pitot probe and clog it.
Best example of this sort of small thing big trouble is the coin in the elevator mechanism that happened a few years ago to a glider (no damages or whatsoever, luckily).
At my club we had a passenger losing an earring while in flight and quite a bit of take apart followed....

A coin is so different from a styrofoam cup that it's hard to see how any comparison could be relevant.

The thing that a styrofoam cup and a coin have is common is being peircived as small, lightweight and irrelevant. The comparison is to point out that when talking aircraft safety no object is small enough not to have to worry about.

I don't see ramp workers wearing hair nets, so a human hair is clearly an object that is small enough not to have to worry about. But I didn't say that either a styrofoam cup or a coin is so small that you'd not have to worry about it: I said that they're so different that knowing about one tells you next to nothing about the other.

You can either choose to understand what my point is (Object that are small and not perceived as an hazard for vehicles people are familiar with, such as cars, may be a problem for aircrafts) or go on nitpicking. Either way, enjoy your day. Oh, by the way, a human hair in a i.e. micro mechanics assembly shop where your avionics are being manufactured may be quite a problem...

A coin is self-evidently dangerous: coins are a lot like stones and stones can clearly cause damage (also, this). Human hairs show that there are some objects that definitely are not a problem. Styrofoam cups aren't obviously safe like a hair or obviously dangerous like a coin. That's the whole point of the question: the answer isn't obvious, so the person asked a community that knows more. I agree with your general message; I just don't think that a coin is a useful example.

@DavidRicherby I would agree with Caterpillaraoz's comments. In the wrong place anything of any size or material could be a FOD hazard, just with different probabilities.

If any engine fails due to a foam cup it was already failing.  Obviously you never want fod on the flight line but there's a significant difference between soft and hard fod.


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How can a "due regard" state flight that has been intercepted indicate to their interceptor that they need to deviate around a hazard?
Military (state) aircraft sometimes conduct observation and intelligence-gathering flights near (but not within) the territorial airspaces of other countries, flying without the express permission of said other country, but in international airspace with "due regard to safety of flight".  Since those other countries are often not on the best terms with the country performing said flight, they will send an interceptor up to meet and "shadow" the aircraft performing said due regard flight until it has cleared the area of concern.  This is all well and good provided nobody does something unexpected or silly-at-best, hazardously-dumb-at-worst.
However, military aircraft are still susceptible to hazards such as severe/extreme turbulence, severe in-flight icing (such as that caused by lofted supercooled large droplets), and thunderstorms, that would force a flight to deviate around said airborne hazard.  While some hazards (thunderstorms) might be visible to both interceptor and interceptee on radar, others (icing, severe/extreme CAT) aren't -- leaving other pilots and their PIREPs as the primary source of information about what's actually going on up there.
So, that brings me to my question.  If the interceptee gets a PIREP saying the hole in the thunderstorm line they planned to thread their way through is closing up, or a bunch of unforecasted icing is up ahead on their route, how can they unambiguously communicate "I need to deviate around this hazard up ahead by X miles left/right" to the interceptor so they don't wind up making the interceptor think they're being noncompliant? Or is this not a concern provided the interceptee's maneuvers stay within a normal maneuvering envelope (i.e. half standard rate to standard rate turns, gradual climbs or descents)?

You say "...so they don't wind up making the interceptor think they're being noncompliant" - but I don't believe the interceptee is under any obligation to be compliant in this situation, nor is the interceptor issuing any orders that they expect the interceptee to comply with.  They're just shadowing.

@pericynthion I'd expect an interceptor to react...negatively to any interceptee making abrupt maneuvers, especially in the case of a military aircraft doing so

Negatively how?  If they were to fire upon the interceptee that would be an act of war, I'm sure their rules of engagement wouldn't allow it.  Indeed it's my (limited) understanding that abrupt maneuvers by the interceptee in such a situation are a common response to such harassment.

Don't they have radio to talk and explain? I expect military pilots being professional pilots speak English correctly, and maybe can key a Q code (ACP 131) with their lights .

I would imagine that climbing, descending, or changing course away from hostile-nation's airspace would be recommended and probably wouldn't generate too much angst, even if the maneuvers were somewhat abrupt. Also, the interceptor will understand the deviation because he will begin experiencing the weather conditions as well.

@UnrecognizedFallingObject if the interceptee is, as you say, outside the territorial airspace of the interceptor, the interceptor has no rights whatsoever to react in a hostile manner to maneovering by the interceptee. This is only a problem in disputed territories like the South China Sea


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What is the most critical takeoff shear margin condition for a Type IV anti-ice fluid?
Type II and the more common Type IV anti-ice fluids use non-Newtonian thickening agents so that they can stay put during ground operations, yet shear stresses on the fluid during the takeoff roll cause the fluid to come off the wings, leaving a clean wing behind that can provide proper lift at rotation.  However, if the fluid itself fails to detach from the wing, it can degrade the aerodynamic performance of the wing just like any other contaminant.
Under what conditions of liftoff/rotation speed, aircraft design, temperature, and precipitation is the margin between "shear forces available to get the fluid off the wing" and "shear forces required to thin and remove the fluid" the least?   You can assume that the fluid in use is a standard SAE Type IV fluid applied as per SAE ARP 4737.

(BTW: if you wonder why I ask -- it's because I'm trying to ascertain what consequences there could be if you anti-iced a slow plane with Type IV)

Under the following conditions, we can't take off:

Temperature below -10 degrees C, and
Small Hail or ice pellets (i.e. METAR "PL" reported), and
Rotation speed below 115 knots

To operate with PL reported, we have to have Type IV anti-ice applied, and under those conditions of temperature + precip, they (yeah, "they") have determined that you need at least 115 knots to be sure the fluid adequately shears off.  There are various ways to drive an increase to your VR speed; it takes a pretty light 737 to rotate at 115 knots.  Other aircraft, of course, might normally rotate well below that speed.
I suspect that this limitation is probably applicable across the US airline industry, but the charts I have are specific to one carrier, so I won't generalize beyond that.
This (PL, -10C) is the only place in all the de-ice/anti-ice charts that we have this 115 knot limit.  With all other forms of precipitation, we have a holdover time and the requirement to visually check the fluid for failure; if it has failed, then we don't take off but rather go back & get it washed off & re-applied.  In the case of PL, there is neither a visual check nor a holdover time, but an "allowance time" after which you have to assume fluid failure & go back to start over.  (With PL, you can't visually identify fluid failure -- so you have a conservative time, and after that you're just done.) Speculating, I'm assuming that the nature of the ice pellets is such that they can, in sufficiently cold temperatures, generate the conditions where the fluid needs a particular speed to shear off that some of our operations might not reach -- thus the 115 knot limit.  Whatever can happen with the other precip, presumably can't keep the fluid from shearing off at whatever our lowest possible VR might be -- which I don't know offhand.  Again, for aircraft with a slower VR, you might get those sorts of situations with combinations of precip & temperature that we wouldn't worry about.
Not a complete answer to your question, but hopefully this data point may be helpful.

So "we" in this answer is "an US based airline", which makes "they" the FAA?

@DeltaLima  Maybe, or perhaps an industry group, or chemists at Dow or several of the Type IV fluid producers, or maybe just guys at our airlne who decided that we could ignore whatever cases required a VR above (values you'd never use in our operations anyway), or maybe some combination of these and perhaps others.  But simply, "they" = the collective group of those who influence the rules we operate under.  Which includes but can be broader than just the FAA.


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What documented practices are there for NOTAM creators and handlers to combat NOTAM spam?
Sadly, NOTAM spam is a persistent thorn in the world aviation community's side.  Bureaucratic rump-covering exercises lead to NOTAMs being issued and distributed that are mere noise to the average airman, leaving them pawing through pages and pages of briefing packets looking for a needle in a haystack.
As airmen, we're largely stuck with the results of this all.  While better filtering would indeed be an improvement over what we have now, it would but paper over the underlying problem: the fact that there's this sea of noise to begin with.
How can NOTAM creators and handlers can this spam problem?  Are there documented practices or guidelines they can follow to reduce the amount of rubbish that gets input into AFTN and dutifully delivered to hapless airmen everywhere?

I see a big game of CYA, if they don't issue a NOTAM and somebody gets hurt as a result, somebody is liable. Personally as a pilot I'd rather read through noise to find things that are applicable to me than miss something because somebody deemed it not applicable.

@RonBeyer -- at some point, there's just too much hay in the haystack and you can't find the needles as a result.  (ouch!) Granted, solutions don't have to involve omitting information altogether -- condensing multiple NOTAMs for the same event (have you seen how many NOTAMs the FAA spews out for a runway closure?) is a possibility I've batted around in my head, for instance...

From a non-pilot perspective, the "NOTAM spam" was an interesting bit of history, and from a human being perspective, I'd say the writer is a bit... peeved.

More on NOTAM spam

It's a real problem. Probably the worst you could do is read a raw DUATS briefing. If your EFB includes briefing support, those will do a slightly better job formatting and organizing everything. If you're flying VFR most of the time like me, keywords like IAP, ODP & SID following an airport identifier are only relevant to IFR, and not information concerning that flight.
Needless to say, the burden is on the airman right now. Maybe if someone passed these abuse cases to a forward-thinking FSDO, they might have a chance of some relief.


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How can one fly a stabilized approach when the final approach path is curved?
Some approaches, like the KMTN VOR/DME RWY15 depicted below, have a final approach segment that is curved (in the case below, the entire leg from FAF to RWY is one big DME arc, but this could also happen in RNAV approaches using Radius-to-Fix instead).  However, it is said that one cannot be in a turn in order for an approach to be considered "stabilized" (this is most often stated in the context of circling approaches, but is still applicable for straight-ins of course).  What does one do when stuck with an approach that cannot be flown within stabilized approach criteria?

A better example of an approach with a curved final segment would be this one (albeit no longer in use).

@Sean -- True.  One could also look to the RNP RF approaches in service in places like KGUC and KRIL for examples, or also to the VOR-DME into KJFK 13L/R...

Perhaps my thinking is now considered outmoded (retired in 1999), but for me the definition of a stabilized approach doesn't preclude turning, but only that the turning (when necessary) is accomplished at a stabilized rate. The idea was to be setup in landing configuration, at your stabilized final approach speed, sink rate, and power and tracking whatever the approach path is from the FAF down to the landing flare.
For tracking DME arcs, we were taught to find that angle of bank that (given our final approach speed) produced the desired turn rate. We would hold that bank, just as we would hold wings level for a straight final approach path.
If the DME started in increase, steepen the bank a little. If it started to decrease, lessen the bank a little.

It may be easier to think of a DME arc as a polygon rather than a circle.
Most of the time would be flown wings level, and with a 14.7 mile arc, there would be a lot of room for correction.  So I would say this would be like any other approach except that the final approach course just isn't a straight line.
As far as missed approaches go, once your turn exceeds runway heading at the correct distance of 14.7 miles one would execute.
One thing I do find interesting however: normally once a pilot reaches full needle deflection on the primary navigation instrument they would execute the missed approach.  A DME arc follows different rules, so I would be curious what the limits would be with a DME reading as a course guide. +/- .5 miles or something along those lines?

That last question of yours is quite worthy of a question of its own on this Stack :)

@UnrecognizedFallingObject: You can simulate that on good luizmonteiro.com page ;-)


text:
Do interceptor squawk codes suppress TCAS RAs?
Some parts of the world reserve a discrete squawk code for use by military interceptor operations (in the NBCAP, code 7777 is used for this).  Does this squawk code, or some equivalent value, suppress TCAS RAs from other aircraft, or is turning off their Mode A/C/S transponder replies the only way an interceptor can avoid triggering a TCAS RA from a TCAS-equipped aircraft?  Is some sort of Letter of Agreement used to keep track of interceptors in areas where transponding is mandatory but interceptors cannot squawk during intercepts, or during intercepts outside of primary radar coverage where the interceptor is unable to squawk?

Closely related: TCAS RA during intercept — what's the correct procedure?

When the question says "military interceptor operations", does it mean fighters practicing dogfighting, or is it talking about a fighter being scrambled to check on a civilian plane acting strangely?

@rclocher3 -- "intercept" is uniformly the latter AIUI.  Practice dogfights (BFM/ACM) are a totally different kettle of fish :)

To avoid an RA, the interceptor would almost certainly turn off at least his Mode C, and to avoid a TA and the concern in the intercepted aircraft of "what is this no-altitude traffic doing closing rapidly on me???" he will probably turn off his squawk altogether before he gets to the last few miles of the intercept.  (There were cases years ago when TCAS was pretty new of intercepted aircraft getting a series of RA's as interceptors -- squawking everything -- closed on them, maneuvering to continue the intercept even as the target climbed and dived all over the sky in response to the RA's. That scenario is undesirable for all sorts of reasons!)
A wingman may well hold a squawk and park himself some miles in trail of the target, monitoring both the interceptor and the target with his own air-to-air radar.  ATC can see the wingman and the target, and knows that there is an interceptor somewhere between them -- which is probably enough for their purposes.
Each specific case probably has letters of agreement between the ATC facility and the specific interceptors, so they'll all know in advance who's where, doing what, squawking or not squawking what, and who's talking on which frequencies.  Those sorts of specifics probably aren't available in the public domain.

As it turns out, it's not keyed off the squawk code after all
Since different regions of the world use different beacon (squawk/SSR) code allocation schemes, having TCAS RA suppression keyed off a special "squawk" wouldn't work so well.  However, there is generally functionality to allow military interceptors to prevent errant RAs from interceptees, or at least that's what ICAO foresees as present in Appendix 7 of ICAO Doc 9863, aka the "master document" national aviation authorities work off of when setting their requirements for ACAS.  Furthermore, there is a difference between the "legacy" (Mode 3/A/C) and the "new style" (Mode S, 1090ES) transponders in this regard, although it likely is implemented in the same way by the intercepting pilot.
For grizzled greybeard fighter jets...
If the interceptor does not have Mode S/1090ES support itself, there are two ways the interceptor can prevent a RA aside from strangling its squawk altogether, which is highly undesirable during peacetime or wherever primary radar coverage is unavailable.  First, the interceptor's transponder can ignore Mode C interrogations entirely, which is useful for covert intercepts, but means that the interceptee's ACAS won't generate a TA or display icon for the interceptor.  For demonstrative intercepts (such as in a lostcomm situation), where a TA is desired but a RA needs to be suppressed, the interceptor needs to generate a Mode C reply containing only framing pulses (i.e. no altitude data) in that case.
As to how that plays out in practice, older F-16s (and older F-15s as well, as they use a similar transponder interface) had the ability to cut out Mode C, but it's not known if that suppresses framing pulses or just the altitude data, while their F/A-18 counterparts transmit framing pulses only (aka sending an altitude reading of 0) when Mode C is disabled.
Things change with this newfangled Mode S tech, though
Modern jets, or older fighters with modern transponders capable of Mode S operation, handle things differently so that they can still provide altitude readouts to ground interrogators (ATC) while suppressing responses to aerial (ACAS) interrogations.  In particular, for covert intercepts, an interceptor's Mode S transponder needs to ignore DF0/DF16 interrogations, while a demonstrative intercept simply suppresses the altitude returned to such an interrogation instead.  The ICAO documentation says this behavior is part of an "Intercept Mode", but I am not sure how it's activated on today's military aircraft; my guess is it's a menu item somewhere, but there could be a dedicated switch or key, depending on the aircraft in question.


text:
Could a GE90-115 engine generate enough suction to ingest a typical passenger vehicle?
We all know that a jetliner can generate enough blast behind it to toss around cars, light aircraft, and even the occasional school bus, and also can generate enough suction to inhale (and promptly destroy) unwary ground crew.
However, if some foolish ground person drove a car (let us assume it's a typical passenger car of exactly 1 ton weight) into the intake hazard area of a GE90-115 engine during a full power runup, would that engine generate enough suction to pick the car up and attempt to ingest it? Or would they have to get so close to the intake that they'd notice that they'd better put the pedal to the metal and get out of Dodge before they went for one last, most unfortunate ride?

1 ton is actually a pretty light car. For example, a SMART car has an 1800lb curb weight (the FourTwo is a little lighter at 1650). I'm guessing though that the answer is "no" because the air is pulled in from a much larger area and lower velocity (and relative pressure) than the outlet.

@RonBeyer -- it wouldn't surprise me if it was a "no" -- but it'd be interesting to see a solid set of calculations showing so, nonetheless.

I think this is bordering on a physics.se question, aside from it being a jet engine, you are asking what relative suction pressure it would take to move a specific mass, and then if the GE is capable of generating that pressure differential at some distance from the inlet.

@RonBeyer -- if you feel it should be migrated, flag it for migration -- I'm certainly not opposed.

Not a car, and no information if it was full or empty, but related:  http://www.seattletimes.com/business/boeing-aerospace/boeing-747-engine-sucks-in-luggage-container/

@Adam The article says "vacuum created by the air intake of the left outboard engine was so strong that it pulled the empty container off a baggage cart...", so it was empty.

That container looks like an AKE can. Even the all-aluminum AKE's don't weigh much more than 200lbs empty. I think the tare wt listed on the ones we use with canvas doors is 82kg

This question would probably be better answered by an automobile aerodynamicist, so be wary of my assumptions. Hopefully, I can at least outline how to get a rough estimate.
Assuming that a "typical passenger car" is less streamlined laterally than longitudinally, let's consider a car crossing orthogonally to the intake airstream of the engine. Assume also that the intake air only generates a force on the car pointed towards the center of the inlet, parallel to the ground, and low enough to not tip the car over (this may not be a good assumption, as crosswinds can generate other aerodynamic forces on a car, especially if it's moving forward or backward).
The problem then basically reduces to overcoming the friction between the tires and the surface on which they rest; let's assume it's concrete. We can start right at the inlet, because if the engine can't move the car there, it can't move the car anywhere in the intake hazard area.
Here's a helpful paper (if I'm interpreting it correctly) that experimentally determined some dimensional sidewind coefficients $K$ for typical vehicles under a somewhat analogous situation. It's a bit odd to use a dimensional coefficient (and thanks to Koyovis for pointing that out; always check your units!), but with some unit conversion we can make it work. Let's take $K=.003(239.6)=.7188\;\mathrm{kg/m^3}$ as a typical value. The side force on our car is thus
\begin{equation}
F=KAV^2\,\text{,}
\end{equation}
where $A$ is the area facing the flow and $V$ is the air velocity. Let's say a good value for the area of a side of a car is $5\;\mathrm{m^2}$ based on this calculation from these numbers.
The mass flow rate $\dot{m}$ of a GE90 at full thrust is $1350\;\mathrm{kg/s}$ and the intake radius $R$ is $1.562\;\mathrm{m}$. Taking a standard air density of $1.225\;\mathrm{kg/m^3}$,
\begin{equation}
V=\frac{\dot{m}}{\rho\pi R^2}=144\;\mathrm{\frac{m}{s}}
\end{equation}
or about 320 mph.
You've specified a mass $M$ of $907.185\;\mathrm{kg}$, so we'll use that. The static coefficient of friction $\mu$ between dry concrete and rubber is around $.75$. So the force we need to overcome is
\begin{equation}
f=\mu Mg=6675\;\mathrm{N\,.}
\end{equation}
Meanwhile, assuming the whole side area of the car is exposed to the maximum air velocity,
\begin{equation}
F=KAV^2=74525\;\mathrm{N\,.}
\end{equation}
So, under our assumptions, the engine has more than enough suction to ingest our car at the inlet. However, I think a more realistic mass $M$ is about $1270\;\mathrm{kg}$, which yields
\begin{equation}
f=\mu Mg=9344\;\mathrm{N\,,}
\end{equation}
which is still plenty below what the engine produces.
So we've determined that at the inlet the engine should be able to suck up a car. But what about the entire hazard zone? We calculated the inlet velocity right at the lip of the nacelle, but the area over which the engine draws air is much, much larger even inches away from the intake, seeing as the hazard zones extend behind it. Thus, at a distance away equal to a typical car's width, the velocity will be quite a bit lower. Let's model the increased suction area as a half-circle with a radius $R_0$ extending from the point on the ground directly beneath the hub of the fan.
First, let's calculate the actual airflow velocity needed to move the 1270 kg car:
\begin{equation}
V_0=\sqrt{\frac{f}{AK}}=51\;\mathrm{\frac{m}{s}}\,.
\end{equation}
Now, we can calculate $R_0$ based on that velocity:
\begin{equation}
R_0=\sqrt{\frac{2\dot{m}}{\pi\rho V_0}}=3.7\;\mathrm{m}\,
\end{equation}
or about 12 feet. The intake hazard zone for the much smaller CFM56 has a greater radius than this. So if we assume that all the air going into the engine comes from the hazard area and neglect the presumably increased hazard area for the GE90 (quite conservative), our car will still be okay unless it is right in the inlet. So, "would they have to get so close to the intake that they'd notice that they'd better put the pedal to the metal and get out of Dodge before they went for one last, most unfortunate ride?" I say yes.
However, because vehicles in jet blast tend to get tipped over and lifted from the ground, a simple statics problem is probably not the best model, but I don't have enough background in automobile aerodynamics to tackle the full dynamics problem.

The value of K=0.03 does not seem right. The article you mentioned uses feet and miles etc. In SI units I would expect the value to be between 0.5 and 1 based on frontal area from sideways direction of the car (flat plate perpendicular to wind has $C_W$ value of 2). The C-value is in relation to a reference area, for long slender bodies like aeroplanes the wetted area is taken and symbol is $C_D$, for short blunt bodies the frontal area is taken and symbol is $C_W$.

Wikipedia lists some values for front side direction of the car which is more streamlined (using $C_D$, confusingly). For toppling over, you could use moment arms, CoG between wheels.

Oh, excellent catch in noting that $K$ is not dimensionless. I've updated my answer accordingly; many thanks.

Are you sure OP meant 1 ton as 907 kg? Couldn't it also be 1000 kg? (As an aside, this is one big reason why I dislike it when people use units that have different definitions in different locations.)

@Koyovis: I don't see where the authors took density into account, so it looks to me like $K$ has units of density based on their formula $K=F/(AV^2)$. Please correct me if you see something else in the paper.

@Michael Kjörling: No, I'm not sure, and it definitely could be 1000 kg. It's not a very realistic weight either way, so I didn't focus too much on it.


text:
Have there ever been accidents due to a vacuum failure that hasn't been noticed?
When the vacuum system fails, the artificial horizon slowly goes back into it's non-vacuumed position. I found no picture on that but it's a quite high angle of bank that is indicated.
Has there ever been an accident because of the failure of the vacuum system? In flightsim, I always needed some time to realize the AH is no more working and adjusted my bank to "fly straight" again.

On my first cross-country flight during training my vacuum pump failed (VFR) and the first thing I noticed was that the artificial horizon was a little low, but the larger telling sign was that the gyro compass was lagging heavily. A quick look at the vac gauge confirmed and by the time I turned around it was a complete failure. I landed and the school confirmed the pump was bad, but gave me the option to continue, which I declined (the instruments were not required).

t does not qualify as vacuum failure, because airliner IRUs use solid-state (vibrating or laser) gyros, but this West Atlantic Sweden CRJ2 crashed on Jan 8th 2016 due to attitude indicator failure—and poor design, because the system had enough information to declare itself failed, but didn't.

There have been many accidents because the vacuum system failed; AOPA has a good article that discusses some of them, and see this NTSB preliminary report for an example.
As for accidents caused because the pilot didn't notice, that's a more difficult question: if the pilot didn't survive then there's no definitive way to know what happened and what he experienced. It's always possible that the pilot was fully aware that the vacuum system failed, he just wasn't good enough at partial panel flying to keep the aircraft under control.


text:
Why weren't the XB-70's wingtips fixed in the down position?
The XB-70's wing tips folded down in flight to:

improve directional stability at supersonic speeds
reduce shifting of aerodynamic center at supersonic speeds
reduce drag at high supersonic speeds

With thanks to Peter Kämpf's answer. Actually, this question was prompted by that answer, and quite a bit of it is 'borrowed' from there.
Wings in flight:

Source: interceptor.com
Landing with wings straight:
Source: Gawker Media
My question is why did they fold, instead of being fixed in the down position?
My assumption is that the plane already sat very high off the ground (probably to give clearance for take off/landing rotation), and lengthening the landing gear enough to allow the wing droop would have been a greater engineering issue than making the wing folding mechanism. Is there validity in that assumption, or, were the benefits of the wing droop at supersonic speeds detrimental at subsonic speed?
Some perspective of just how tall the XB70 was (click to zoom - that's a tall bird!):

Source: cybermodeler.com

Look at how far the wingtips extend downwards past the engines in the in-flight picture, and then look at how close the engines are to the runway in the next picture.

A confirmation of my assumption, @fooot? aeroalias' answer indicates there are aerodynamic issues, as well.

Yes, just pointing out that landing gear would have to be even longer than is apparent when it is level.

FWIW, the XB-70 wingtips folded down to exploit a phenomenon called "compression lift"

Thanks, @AnthonyX, as mentioned in the question, I understood the value of the drooped wingtips, the question was why did they fold instead of being permanently drooped.

Well, for starters, the drooped wing is not producing any lift in subsonic speeds. So, the flat area (the fixed part) should be significantly larger for producing the lift. This increases weight, which leads to a vicious cycle.
Another thing is the wing would panels would not clear the ground while the aircraft was on its landing gear. For testing the wing fold in ground, the aircraft had to be put on tall jackstands.

Image from keypublishing.com
As you noted, lengthening the landing gear enough to allow the wing droop would have been a big issue not the least because the aircraft weight was already increasing alarmingly (the weight had shot up from 483,000 lb to 537,000 lb).
Also, you lose the outboard elevons; now for getting the same control response the inboard control surfaces have to be increased in size, which has to be come either from wing or increase the wing area- it is again a mess.

Thanks for finding that shot of the wing fold check. I worked at North American 1960 -1962, and coded the Fortran software that analyzed the static-solution loads off the wind tunnel data for the six hydraulic hinge actuators in each wing that moved the wingtips.

@Terry, seriously, will you be my grandpa so I can sit on your knee and you can tell me all your stories? I come complete with a wife and kids who will soon be producing great-great-grandkids for you! I've rummaged around your website a bit - it would be awesome if you would just start documenting all these stories there.

First, you need more wing area at low speeds in order to have a reasonable landing speed.  Second, you want downward turned wingtips at supersonic speeds to create compression lift.  At supersonic speeds the center of lift moves aft making the airplane pitch down.  You have to trim this change away and that creates drag.  In the case of Concord you pump some fuel to the back of the airplane to keep everything in balance but on the XB-70 you eliminate some wing area on the back of the airplane by turning the wingtips down.  So at subsonic speeds you need the wingtips to create lift but not at supersonic speeds.

The purpose of the pivoting wingtips, which could be rotated downwards to an angle of 65 degrees from the horizontal during supersonic flight was to improve directional stability, while shifting the centre of lift forward to a more favourable position for high speed flight, and at the same time helping the compression lift effect by confining the shockwave generated by the sharp vertical central splitter plate of the air intake between the downturned wingtips, thereby creating a positive pressure area underneath the wing. This use of the compression lift principle made possible a 30% reduction in induced drag at high speeds. The XB70 in supersonic flight was said to "ride on it's own shockwave".

Thanks for that. As noted in the question, I understood the purposes of the droop. The question, bolded in the OP, was why the wings were movable instead of being fixed in a droop. While adding a couple of extra benefits of the droop, unfortunately, your answer doesn't address the question at all.

Having the wingtips fixed at a 65 degree angle might have made takeoff and landing hazardous I think. Take another look at the photos above, and you may see what I mean...

Having the wingtips fixed at 65 degrees down would have necessitated extremely long main and nosewheel gears, which would have made landing, which was already quite demanding due to the height differential between the nosewheel and the cockpit, even more difficult. So it made more sense to design the aircraft with folding wingtips.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What switch was Skiles talking about shortly before the Hudson landing?
I'm always amazed by the "miracle of the Hudson River landing" and just now stumbled across this video by the NTSB.
At 3:52, Jeffrey Skiles appears to say "* * switch" shortly before they ditch in the Hudson. I am assuming it was unreadable on the recording what he said before "switch". Does anyone know? Any ideas what and why he said that?
Side question: On the video it is not identified who is speaking but rather what channel it was spoken on. HOT-1, I assume, is the captain, HOT-2 the first officer. PA and stuff is obvious, but what is CAM, why did it stop recording on impact and why does the first officer sometimes speak on CAM, sometimes on HOT-2?


text:
What happens if an A320 goes to full throttle with the parking brake set?
The question is simple: what happens?
Is the aircraft going to stay stationary, or will the engine power overcome the brakes? I'm assuming everything is fully functional.
I know the FCOM says not to exceed 75% N1 with the parking brake engaged, but if we did, what would happen?

Different question, with different answers.

@DanieleProcida but OP themselves said that the duplicate answers their question.

@Federico Where does it say that?

@DanieleProcida the fact that the "community" account has closed the post. It only happens if the OP agrees on closing the question as duplicate.

The question is simple: What happens?

There is a strong chance this happens:

Credit: spiegel.de

On 15 November 2007, an Airbus A340-600 due to be delivered to Etihad Airways crashed during ground engine test at Airbus' facilities in Toulouse Blagnac International Airport. The brand new US$250 million aircraft, damaged beyond repair. was written off

Source
And there is a stronger chance that nothing bad will happen, but when you exceed the limits clearly stated by the manufacturer of the equipment without any good reason then your insurance cover flies away faster than the exhaust air so if Airbus tells you to not exceed 75% N1 then it is safe to assume that it is not safe to go 76% N1 on both engines with Parking brakes set.

Wow. Talk about a bad day at the office.

The A320neo has maximum combined thrust of about 60,000 pounds; a minimum operating weight of 97,000 pounds; and maximum takeoff weight of 174,000 pounds. Typical modern tire rubber on dry pavement has a static friction coefficient of 0.9-1.1
0.9*97000=87,300 pounds of thrust needed to make the tires slide.
The engines would get very hot for sure.
The airflow may pick up debris and damage the engine.
Hot exhaust may deflect off the ground and heat the tail?

I'm confused. What would the amount of force required for the tires to slide (assuming the aircraft is completely unloaded with no fuel) have to do with how hot the engines would get? Airliner engines are designed to get hot. Very, very hot. Like, "Magma looks cool by comparison" hot.

Up until the talk of engines getting hot I thought this was a good answer. You may want to edit that out.

The engines are not designed to operate at full load for more than a few minutes, typically only for emergency maneuvers like an engine going out just after rotation, normal takeoffs are only 80-90% throttle. The engines are also not designed to operate at full load while static, they need to move forward for cooling and to prevent compressor stall. Compressor blades have an AoA just like wings. Turbines have temperature limits, maximum material temperature is the number one limitation on turbine design (power and economy performance) sitting at full power on the ground will melt the parts.


text:
How would SLP997 be calculated on a METAR?
How would SLP997 be calculated on a METAR? I understand that SLP117 say would be 1011.7 hPa, but I am sure that SLP997 is not 1099.7 as that would be impossibly high.

Refer to FAA document, AC 00-45H Aviation Weather Services

3.1.5.13.25 Sea-Level Pressure.
At designated stations, the sea-level pressure is coded in the
  following format: the identifier SLP,
  immediately followed by the sea-level pressure in 1 millibars (mb) .
  The hundreds and thousands units are not coded and must be inferred.
  For example, a sea-level pressure of 998.2 mb is coded SLP982. A
  sea-level pressure of 1,013.2 mb would be coded SLP132. For a
  METAR, if sea-level pressure is not available, it is coded SLPNO.


text:
Why do large turbofans generally have many more LP turbine stages than HP turbine stages?
In almost every large turbofan I see, the LP turbine stages outnumber the HP turbine stages by a factor of at least 2. Here's a photo of the RR Trent 900:

The Trent 900 (like most Rolls-Royce turbofans) is a 3-spool turbofan, rather than a 2-spool. But even here, my observation holds. There are far more LP turbine stages than the middle or high pressure stages.
Why is this?

Because the LP turbine extracts power for the fan, which requires the most power. The HP and IP turbine only extract power for their connected compressors - the LP turbine extracts power for the fan and the LP compressor. The fan does work on all airflow through the engine, the compressors only on a fraction (10:1 for a high bypass like the Trent 1000). The fan produces up to 75% of the thrust of the engine.
From this presentation: the cross section of the Trent 1000. It shows that the LP, IP and HP rotors have different rotational speeds, but does not list them. Another (older) presentation lists them as 3600. 6800 and 10200 RPM. Rotational speed goes down as the volume of the mass stream goes up.

An old textbook of mine gives the power P extracted from a turbine stage as:
$$ P = \dot{m} \cdot u \cdot v_{ax} \cdot [tan (\alpha_2) + tan (\alpha_3)]$$
with

$\dot{m}$ = mass flow [kg/s]
u = tangential blade velocity [m/s]
v$_{ax}$ = axial gas velocity [m/s]
$\alpha_2$ and $\alpha_3$ angles according to the figure below.

So the tangential speed of the turbine blade is in the equation for power extraction, which is composed of rotational velocity and blade radius. The faster the turbine turns, the more power can be extracted per stage, and the fewer stages required. Why then does rotational velocity go down with pressure? (HP = 10,200; IP = 6,800).

The reason is the constructional limits of the turbine. As the gas stream expands, the turbine blades become larger and are mounted at a greater axial distance, which results in greater centrifugal forces which are proportional to blade mass, rotational speed and distance from the axis. In order to limit centrifugal forces, the rotational velocity of the rotor is reduced: each subsequent stage turns at a lower RPM. Notice that a lower RPM can be compensated by mounting the blade further away from the rotational axis.
The LP rotor of an un-geared engine runs at the same rotational velocity as the fan. Its optimal RPM for the turbine may be higher, and that can be accommodated by the geared fan. As the bypass ratio gets higher, the LP turbine will be extracting a higher fraction of total power from the airstream - turboprops and turboshafts have a gearbox in between the LP shaft and the propeller/rotor, and the high bypass fan approaches the relative dimensions of a propeller...

Interesting but in this case, with 3 spools, this begs the question of why not balance the stages more? Like put 1 or 2 for the fan on the first spool, 1 or 2 for the smaller LP compressors, and 1 or 2 for the HP compressors. Instead the LP compressors have by far the most stages and are only powering the fan, which to my knowledge is just the lowest of the LP compressors.

The IP and HP spools spin faster than the LP spool.

@DrZ214 That's the very point of a multispool engine: to have the fast components on fast spools and slow component on a slow spool. What you propose is pretty much single-spool engine, proven to be not the best solution. Turbofans are difficult to understand, because the difference between fan and compressor appears blurred. Start at turboprop or shaft engine, there you get clear-cut distinction between driving fast-spinning internal components (compressors) and a slow-moving external component. Fan is little more than a prop mounted in same casing. Fan is not a compressor stage.

You might want to add that the HPT is working in much denser air. For a compression ratio of 40:1 the density change is about 14. If the last stage of the LPT expands air down to ambient pressure, the HPT needs only 7% of the blade area for the same power extraction.

Thanks for the equation, it should prove useful. But what exactly is u? (radial blade velocity?) The blade should not be moving in the radial direction. The blade's radius should be constant. Could it be tip speed?

:) Yes indeed, the radius should be constant. It's the tangential speed of the blade at mid point, taken as the average, have corrected.

The answer provided by Daniel Kiracofe is correct, I just have a couple of additional comments and I don't have enough reputation to add this as a comment to his answer.

On high bypass engines, the speed of the LP rotor is usually limited
by the tip speed of the fan due to its large diameter.
One of the first determinations in choosing between a geared and a
non-geared (conventional ) design is to make sure you can remove at
least as much weight from the LPT as you are adding with the
gearbox. If you can't, then the geared design is not a viable option.

Can you provide a quantification or a reference for your claim?

Here's an even more interesting question. Take a look at the P&W geared turbofan cross section (e.g. this article).  It only has three LPT stages!  So how come the GTF has 3 and all of the other engines like the RR Trent 900 have a lot more? (e.g. a GE90 has like 6 or 7).
The answer is this: fans are best when they run slowly. LPTs are most efficient when they run quickly.  On an engine like the Trent 900 or GE90, the fan and LPT are on the same shaft, so they have to run at the same speed.  So you have to compromise. Usually the compromise tends to favor the fan. So, because the shaft is running at the speed that the fan wants to run at, the LPT is inefficient. That is: each LPT stage is not extracting much power from the air.  Therefore, to get out the total power that you need, you have to have a lot of LPT stages.
Now, on a geared turbofan, there is gearbox between the fan and LPT.  That means that the fan and LPT don't have to run at the same speed.  The fan can run slowly like it wants to, and the LPT can run fast, like it wants to.  In that case, the LPT is much more efficient, and you need only a few stages.

The geard turbofan still requires 3 stages, as opposed to 1 stage for IP and HP..

And an even more interesting fact: the P&W geared turbofan is used on the A320Neo. The Trent 1000 powers the B787, a much larger aircraft. Much more power to extract for the much bigger fan.

In addition to Koyovis' answer which discusses the output requirements of the turbine stages, there's an input issue as well:
The HP turbine stage has the best spot: it has the highest input pressure, so you only need one stage to extract enough power to run the HP compressor.
The LP turbine stages run on exhaust gases that have already had some energy extracted from them, so lower pressure->stage is less efficient->you need more of them to get the required power.

I like some of the answers here, but none seem to answer the question from the fluid mechanics' perspective.
One of the reasons the low pressure region has more stages is because you want to avoid flow separation which cause a loss of energy to recirculation instead of providing thrust or power. The low pressure region is compressing; this causes an adverse pressure gradient. In layman's terms, the flow basically wants to go backwards because high pressure tends to flow towards low pressure.
The turbine blades have airfoil shapes, and they work exactly like aircraft wings in that lift is produced in one rotational direction to produce a desired effect. Therefore, if you try to turn the flow too drastically without considering the adverse pressure gradient, then you will get flow separation and a drastic drop in your compression ratio and a drop in efficiency for all of the other turbine parts.
Turbomachinery designers want to turn the flow a certain amount, but they do not want to do it with one stage. So, the engineers opt to turn the flow a little bit each during each stage, and it turns out that the extra weight from all of that metal is worth it.
On the flip side, the high pressure section has fewer blades because the flow really wants to just leave already, so recirculation is much less likely. This is where you can cut corners and put half as many stages. Look how drastic the flow turning is on this high pressure turbine blade here. http://www.technology.matthey.com/wp-content/uploads/articles/39/3/pmr0039-0117-f2.gif

See how the low pressure blades are almost straight?
https://en.wikipedia.org/wiki/Flow_separation

that watermark makes me think that you cannot post that image here.

For Two specific reasons.

Higher power off take - LPT drive LPC and Fan together in 2-spool and GTF architecture (and only Fan in 3-spool architecture). In any case the Fan alone or Fan + LPC will take up significant portion of the total work input. Thus high mechanical power needs to be extracted by the LPT.
Lower RPM: But perhaps the more important factor is except in GTF, the LPT are run at very low speed compared to the HPT at which they work sub-optimally. Thus energy extraction per stage is reduced significantly. Hence more number of stages needed. As already pointed out in another answer, since GTF removes this restriction, the PW1000G engine needs only 3 LPT stages as now they rotate at ~10000rpm instead of ~3500rpm for other engines.


text:
When were flares first used as IR countermeasures, and by whom?
I've been trying to find the history of flares (as countermeasures to heat-seeking missiles), but have not had much luck. Closest thing has been this wiki article which has but a single sentence in its history section saying "First deployed during the Vietnam War", with an explicit "citation needed" tag.
Another article is also surprisingly void of history details, and doesn't even have a history section.
Who first conceived of flares as IR countermeasures? When and where were they first used on a large scale?
The only hint I have is that the AIM-4 Falcon was the first IR-seeking AA missile to enter service, doing so in 1956. So presumably flares would not have shown up until that year or later.

The document Genesis of Infrared Decoy Flares- The early years from 1950 into the 1970s by Naval Surface Warfare Center has a lot of details about the work done on decoy flares in the initial stages, at least in the US.
According to the report, the first use of IR flares was not as a decoy, but as a tracking device for drones. The T-131 flare was used in this capacity. From the report:

NOTS, China Lake was involved with infrared target augmentation since 1954. ... The T-131 tracking flare developed at NOTS was the first infrared source for drone use. Six to sixteen of these are needed on the F6F-5K drone to provide a suitable signature.

The first IR flare developed as a countermeasure is the NOTS Model 704, which was developed around 1959. However, the studies on decoys for IR started in the mid fifties, though the concept of using flares came later (the initial concepts were towed decoys). From the report:

The Hayes Corporation, Birmingham, Alabama initiated infrared countermeasures and radiation suppression studies in 1955. ... There was a need about 1957 to create a decoy device that would simulate the signature of a turbojet engine. Instead of the pyrotechnic decoy flare approach,
  ... proposed an inflatable envelope that absorbs the energy from a self-contained pyrotechnic and re-radiates this energy at a lower temperature approximating that of the heated surfaces of the turbojet engine.

Initial flares (RR-77, probably) were fitted in the B-47 and B-52 bombers, which were deployed through the AN/ALE-14 dispenser around 1960. Though the report has indications that the flares were used in the Vietnam war, there is no data on the exact time and location of the use of flares in combat. However, we can be sure that the IR flares were used in combat in Vietnam. The report says,

In 1967, Dr. Edwards of Air Development Test Center at Eglin Air Force Base, requested Picatinny Arsenal to improve the ALA-17 flare. The Air Force wanted a quick fix to the poor rise time of the ALA-17 flare. They also required that a single ALA-17 flare would have decoy capability to protect B-52 bombers in Southeast Asia against seekers operating in the 2μm to 3μm and 3μm to 5μm bandpass.

indicating feedback from combat operations. Though I'm sure that the Soviets too were developing IR flares at the same time, not much detail is available about their effort.

My father commanded a Stinger AC 119 squadron in SE Asia in the early 1970s. I don't recall the exact dates except that he was there before May 1972 and retired from there in about March 1973. He would tether airmen outside the planes so they could spot incoming heat seeking missiles and fire off flares to draw the missiles away. Eventually, the Air Force replaced this primitive but effective deterrent with equipment that did not require men to fly while hanging outside the plane. You can imagine how scary that would be!
My father is Lt. Col. Albert W Brow, retired.


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Why are trailing-edge control surfaces usually split?
I've noticed on many military and commercial jets, the trailing edge control surfaces (ailerons and flaps) are separate from each other.
Why is this? Can't both serve the same function? It would save weight and complexity if one combined control surface ran the length of the trailing edge, instead of being split in half.
A particular example I found is the F-102 Delta Dagger. There's no conventional tail to experience disrupted airflow, so why have separate ailerons and flaps?

A similar idea that I know exists is the V-tail, such as in the F-117, which has "ruddervators"---combined elevators/rudders. It can still control yaw and pitch at the same time by somehow averaging the deflections. Presumably, combining and averaging ailerons and flaps should be even simpler, since they're both deflecting on the same axis.

There is no "usually" here. For every aircraft with split control surfaces you come up with, I believe it would be possible to come up with an aircraft that either has one combined control surface (eg F-16 has flaperons), an aircraft that has some kind of a combination (eg F-18, which has an inboard flap and an outboard flaperon as can be seen here - warning headphones users -

)
or even an aircraft that doesn't even use ailerons (eg F-14, that only uses elevons and spoilers on the top of the wing for roll authority).
What you may consider to be less complex structurally might end up being more complex as much as the Fly-By-Wire software logic is concerned.

What you may consider to be less complex structurally might end up being more complex as much as the Fly-By-Wire software logic is concerned. Well okay but I'm pretty sure saving weight and moving parts at the exchange of a more complex software is acceptable these days with the kind of computers we have. For the record, I wasn't thinking of anything with an all-moving tail. I wasn't thinking of elevons. I was thinking of flaperons or any long control surface on the wing itself, because presumably the wing is big enough to hold a large combined control surface.

It would save weight and complexity if one combined control surface ran the length of the trailing edge, instead of being split in half.

If the trailing edge real estate is there, it makes sense to split control surfaces along their functionality. The issue is maximum deflection: at full flaps, a flaperon cannot command any additional aileron down input. Combining two or more surfaces poses issues such as identified in this answer.
Having said that, combining control surface functionality and applying computer controlled functionality means that the flight controls can be reconfigured, in case of battle damage for instance. That would increase weight and complexity, and also redundancy and survivability.


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Why are some runways apparently wobbly?
In this youtube video, there are several examples at 0:34, 1:15, and 1:27. You can see the runway looks wobbly. That last one appears to have at least three wobbles in it.
These are runways on which large commercial jumbo jets are landing, so presumably they're built to world-class standards. Do these standards allow for such wobbling along they're length? If so how is that possible?

Note also that those shots are taken with very long telephoto lenses, which make perspective look very compressed. The hills on the runway at 0:34 look very steep but, in reality, they're much shallower.

There are a lot of tricks of perspective here, the runway looks like its barely 1000 feet long, but in reality 15/33 at BHX is 8527 feet (over a mile and a half). It also looks like a lot of the aircraft either hover, come down fast, or rocket into the air but they are probably pretty close to the 3° glideslope.


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What is the actual exhaust velocity of the exhaust from the J58 engines at cruise (Mach 3.2)?
The SR-71 ran on two J58 turbojet engines and cruised at Mach 3.2.
What I want to know is, what is the actual exhaust velocity (relative to the airplane itself) of the jet exhaust at those conditions?

The is a good question, I cannot reply to you but as mach speed depend on temperature it might be a lower mach number.

It's not a cut and dried single figure but  this
SR-71 PROPULSION SYSTEM - P&W J58 ENGINE (JT11D-20) seems to have enough information to allow you to arrive at a reasonable figure with 'some work'. If I had to choose one information source to get a feel for the answer this would be on the short short list (so far :-) ). Unlike most other sources, the result will be at least somewhat meaningful, if not overly accurate.
It also seems to make it 'clear enough' that if someone gives you a simple unqualified numerical answer it will be meaningless, or wrong, or both :-). It's a 44 page PDF slideshow - but has enough detail to show how different the J58 is to what you may expect.
He also supplies these references which will almost certainly be interesting, and possibly also useful.

Connors, Jack, The Engines of Pratt & Whitney: A Technical History, AIAA (American Institute of Aeronautics and Astronautics, Inc.), Reston, VA, 2010, pp. 321-332.
St. Peter, J., The History of Aircraft Gas Turbine Engine Development in the United States: A Tradition of Excellence, ASME, New York, 1999, pp.264-275.
Campbell, David H., F-12 Series Aircraft Propulsion System Performance and Development, AIAA No. 73821, 1973. Also found in AIAA Journal of Aircraft, Vol. 11, No. 11, November
1974, pp. 670-676. Note: This paper presents information only up to Mach 3.0 due to security restrictions at that time.
California Institute of Technology, Course Ae107, Case Studies in Engineering: The Lockheed SR-71 Blackbird, Spring 1991. Published by Cal Tech for course students and presenters only.
Anderson, J. Thomas, unpublished training course syllabus, How Supersonic Inlets Work: Details of The Geometry And Operation Of The SR-71 Mixed Compression Inlet, 2013.

If you do want a single misleading and probably wrong figure :-) then this book say 18,587 m/s at Mach 3.2 (table 1 page 175)

More M&PW answers:
Page 39-40
and here
Somewhere in here probably :-).
Related;
Comment in passing bottom of page 1
and
Here

In the book by St. Peter, I presumed the word Tradition doesn't appear twice in the title, so I fixed that. Excellence could appear twice, but probably doesn't so I tweaked that, too. Please feel free to revert if I've overstepped bounds. Also, +1 for providing a "single misleading and probably wrong" figure to answer the question - most excellent! ;)

@FReeman You're very welcome, Sir, he said. Thanks I looked at the original and it had no duplicates. Also ref 5 had 2 duplicates!. I tried re cccccopying and ppasting and it dddddoubled up again the same way ! Something is fooling its system. So I hand corrected ref 5 and have now added an Amazon link for ref 2. (Yessss the repeated characters in this comment are intentional :-) ).

18.6 km/s? That sounds more like an effective exhaust velocity (considering only fuel burn and not the mass of oxygen mixed with it) rather than the actual exhaust velocity.

@DrZ214 Yep  - that's what you get from a "... single misleading and probably wrong figure..." :-). The first reference and some figuring may be as good as you will easily get.


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Would the "AOA Disagree" light be the only way 737 MAX pilots could detect malfunctioning AoA sensors?
Or would there be other warnings on the PFDs?

I'm not sure how this is "under investigation".  All you need to answer this question is enough experience with the 737 MAX to explain how AOA sensor values and failures are annunciated. Sure, the specifics of the crashed plane aren't easily available yet, but the OP is asking about AOA mechanics in normal conditions.  We have many other questions about 737 Max AOA that are not being closed.

If there was an AOA gauge, then looking at that could have told him there was a problem without an AOA disagree light.
It'd be better than the AOA disagree light too. If the same mechanic maintained both AOA probes or their signals were sent through the same wire harness bundle, they could both agree on the wrong answer.

According to national news sources, neither of the two MAX had the AOA gauge (indicator) or the disagree light. See the link in the original post. If the indicator were to show the same signal that the MCAS was using, that'd be the information needed to know systems depending on AOA were going to be weird. Odd that both aircraft had AOA indicator issues, I flew thousands of hours and were around tons of A/C with AOA sensors, and never knew of a failure of one. To have two like this in a short timespan...


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Why do some aircraft have symmetrical tip airfoils while others do not?
On some aircraft, the tip airfoil is symmetrical, while others carry their root foil all the way through the tip. It would seem an aerodynamic disadvantage to have a non-symmetric airfoil at the tip since it is not making much (any?) lift.
It doesn't seem to be simply for manufacturing purposes since it is on planes with tapered wings as well as straight wings.
Why is it incorporated into wing design?

Camber shifts the airfoil's section of lowest drag of the drag polar to higher lift coefficients. In order to create the least amount of drag in level flight, an airplane wing benefits from moderate camber. Most of the lift is created in the mid section, but enough lift is left towards the tips to justify camber there, too. In case of elliptical or tapered wings, the design lift coefficient for lowest drag at a given lift will be equal over the whole span (elliptic wing) or even peak near the tip (highly tapered and untwisted wing with no regard given to stall behavior or spar mass).
Normally the tips carry less load than the inner wing, details depending on wing taper and twist. This helps to prevent the tip section stalling first at high angle of attack. Good aileron effectiveness also requires an airfoil which equally produces positive and negative lift at low drag.
One example would be the Me-262. When Ludwig Bölkow designed the wingtip, very little was known about transsonic effects and a thin, symmetric airfoil with an elliptic nose was chosen because it showed a late onset of transsonic effects in wind tunnel tests.
When North American later designed the F-86, the outer wing design of the 262 was copied over because it had worked well in the German design.
Many Horten flying wings used symmetric tip airfoils because the bell-shaped lift distribution needed for stability resulted in a slightly negative lift at the tips at low angle of attack and very little positive lift at high angle of attack.
With some designs I am sceptical that a symmetric airfoil has been used even if it is claimed so in David Lednicer's outstanding collection. The Lockheed P-3 is there listed with NACA 0014 at the root and NACA 0012 at the tip - no, the Lockheed engineers 60 years ago knew better than that!

Thank you for the response. I should have been more clear in my question. I understand the use of symmetrical airfoils at the tips, just not for cambered airfoils. Could you please elaborate as to why positive camber at the wingtips is beneficial aerodynamically?

@casousa Positive camber reduces drag for positive lift coefficients. Even near the tip wings still produce enough lift to justify the selection of a cambered airfoil.

Generally you want a slightly less effective AOA at the tip to keep the ailerons working near stall. A symmetrical section would do this if the root were flat bottomed. You wouldn’t lose as much cruise speed compared to washout.

I was told by a retired engineer from Cessna that the reason the Cessna 182 had a symmetrical airfoil at the wingtip, at least on early models, was indeed so that only one version of the wingtip fairing needed to be manufactured, rather than separate left and right versions.

Symmetrical airfoils do not cause lift at all, because there is no pressure difference.They lift just because of the angle of attack.They are profitable on private jets because there is enough speed and the weight isn't large, so, symmetrical airfoils is enough.That's maybe not right, but it's my opinion.


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Where can I find official definition of VFR and IFR?
I know that ICAO Annex 2 'Rules of the air' states rules such as general rules, visual flight rules, instrument flight rules, etc. And aircraft should fly in accordance with either visual flight rules or instrument flight rules, and that is VFR and IFR. If aircraft fly in accordance with visual flight rules, that must be VFR.
But apart from these rigid rules, I image VFR as a flight that should fly with outside visual reference and have responsibility to avoid and separate with other obstacles or aircraft and IFR as a flight referring to instrument to avoid and be separated with other obstacles.
I tried to find more kind explanations(rather than ICAO ANNEX 2) about it like I stated above, but failed. Of course, I could find many documents or information like wikipedia, but there was no official source such as ICAO or FAA documents.
Could you give me some source that define VFR or IFR like as I stated above?

To be clear, you're looking for the meanings of Visual Flight Rules and Instrument Flight Rules, not the meteorological conditions that lead one to pick one over the other, correct?

This may be a nice summary of what you are looking for. In terms of the FAA there are lots of regulations that define how VFR and IFR operations occur, what you are aloud to do, what ATC will provide to you, what kind of certification you need and what the aircraft must be equipped with to conduct such operations. Its not one specific regulation so much as a combination of many.

@Pondlife, that question's answer basically points to the ICAO annex 2, which is mentioned in this question as not understandable for the asker, so I would consider it not a duplicate.

@JanHudec I see your point and perhaps the question is just unclear. The OP asks for "official" sources like FAA and ICAO but isn't very clear on why they aren't "kind" enough. It might be helpful for the OP to explain exactly what's unclear from the Annex 2 rules, otherwise I'm not sure what we can answer exactly. Just my two cents, as always.

I'm sorry for those of who feel my question is duplicate. The reason of why I ask similar question is I was failed to find official data that states VFR as a flight that pilot have to see outside and responsibility to separate with other aircraft or obstacles. Many unofficial information such as Wikipedia, personal blog, etc say it but not FAA or ICAO documents.  Of course, I know that VFR should be operated in VMC but no documents directly state that VFR flight have responsibility to separate by itself.

@Min This question might be helpful. Responsibility for separation also depends on the airspace class, not only the flight rules.

tl;dr The basic differences between IFR and VFR involve weather, required equipment, filing flight plans and getting a clearance, and the altitudes flown.
Your question indicates that you may not know that there is a difference between Visual Meteorological Conditions (VMC) and Visual Flight Rules (VFR). Likewise between Instrument Meteorological Conditions (IMC) and Instrument Flight Rules (IFR). You can be flying under Instrument Flight Rules in VMC—the airlines do this on most flights. It is not wise to fly VFR into IMC—a huge percentage of general aviation accidents happen because of that.
One source of the definitions is the FAA publication Pilot Controller Glossary, although they sort of presume you already know the concept.
INSTRUMENT FLIGHT RULES− Rules governing the procedures for conducting instrument flight. Also a term used by pilots and controllers to indicate type of flight plan.
INSTRUMENT FLIGHT RULES[ICAO]−A set of rules governing the conduct of flight under instrument meteorological conditions.
INSTRUMENT METEOROLOGICAL CONDITIONS− Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling less than the minima specified for visual meteorological conditions.
VISUAL FLIGHT RULES− Rules that govern the procedures for conducting flight under visual conditions. The term “VFR” is also used in the United States to indicate weather conditions that are equal to or greater than minimum VFR requirements. In addition, it is used by pilots and controllers to indicate type of flight plan.
VISUAL METEOROLOGICAL CONDITIONS− Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling equal to or better than specified minima.
The FARs only indirectly define IFR but you can get the rest of the definitions from the FARs.

§1.1   General definitions.
  IFR conditions means weather conditions below the minimum for flight under visual flight rules.
§91.155   Basic VFR weather minimums. (a) Except as provided in paragraph (b) of this section and §91.157, no person may operate an
  aircraft under VFR when the flight visibility is less, or at a
  distance from clouds that is less, than that prescribed for the
  corresponding altitude and class of airspace in the following table:

The table lists the visibility and clearance from clouds in different kinds of airspace that you need in order to fly under visual flight rules. If the weather is less than the minimums, then you need to fly under Instrument Flight Rules.
You can get an idea of the differences by reading CFR PART 91—GENERAL OPERATING AND FLIGHT RULES Subpart B—Flight Rules

Thanks for your sincere answer. Now, I know VFR, VMC, IFR and IMC. VFR 'are' set of rule's' pilot should comply. I'm looking for source that VFR pilot is responsible to see outside to avoid collision. Could you let me know that source?

@Min All pilots, whether flying IFR or VFR are required to see-and-avoid other aircraft. CFR §91.113   Right-of-way rules: Except water operations. (b) General. When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft.

THANKS!!! It's what I am looking for!


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What is the heat signature of a piston engine, relative to a jet engine?
I'm reading about the AIM-9 Sidewinder Missile again. I noticed this interesting photo:

Notice, that's a piston-engined aircraft, and a small one at that. So I'm asking myself, how did the missile lock onto it? The AIM-9 is a very early heat-seeking missile (~1956). I thought early heat-seekers could only lock onto a jet engine, which I believe has a much larger heat signature than a piston engine.
Now I know today that IR missiles can lock onto almost any heat signature, even the heat of the adiabatically compressed air at the wings' leading edge. But in 1956, I'm surprised that an IR missile could lock onto a piston engine's heat.
So I want to start with this question: What is the heat signature of a piston engine, in terms relative to a jet engine? 2x less? 3x less? Or somehow equivalent?

A) Please explicitly source your photo, B) Great photo!, C) Interesting that the missile hit the rear of the plane and didn't go for the hottest part. (Unless, of course, the engine is exhausted from the rear, but I doubt that for a piston engine.)

@FreeMan That is an F6F-5K Grumman Hellcat and you can see from this image that the exhaust is vented out of the cowling underneath the aircraft (also looks like one on the side). Considering the missile isn't precision, it only needs to lock onto a heat source and get near it to explode. The original source of the OP's photo is Wikipedia on Sidewinder Missiles

I don't know where piston engines typically exhaust their heat, but I do know AA missiles usually have a proximity fuze of some sort. Once it flies past the empanage, a microwave sensor detects the object and boom.

@RonBeyer - interesting, I didn't know that about the Hellcat. Also, I found the image source, I was hoping that DrZ would come back to mention it him(?)self, thus my edit for an explicit sourcing. No big deal.

About the missile "hitting" the rear of the plane: Missiles miss. If they actually made contact with the plane they were trying to destroy, they'd be called "hittiles". The missile just needs to get close enough that the frag sphere of its explosion causes damage to the target. On such an early missile, the analog seeker would be pointing the missile at the heat signature, meaning it would fall behind a little as it closed to the target. Newer electronic seekers are programmed to track motion and lead the target to avoid falling behind a maneuvering target.

Actually, it seems that there is a flare attached to the tail. Perhaps the engine exhaust and/or radiant heating was insufficient.

@acpilot Interesting, but what do you base that on? In the first photo, that trail looks like it might be a contrail to me.

It just looks to originate from a "bright spot" behind the tailwheel.  I would expect a contrail to he much thicker and not peter out at the edge of the frame.

@DrZ214 - microwave sensor detects the object It's actually an IR laser proximity fuse, which perform better than earlier passive IR fuses.

@HephaestusAetnaean Are you sure they used IR laser proxy fuse in 1956? Lasers (and masers right before them) were only just invented and, i imagine, very expensive. Makes more sense to me if they used the same type of thing used on VT shell fuses: http://en.wikipedia.org/wiki/Proximity_fuze

@DrZ214 - the AIM-9B (depicted above) used passive IR fusing. Later variants had microwave/radio and even magnetic influence (!) fusing options. The latest variants are active IR (laser).

Here is a color IR image of what I believe to be another F6F, on the tarmac shortly after landing:

As you can see, the engine cowling is very noticeable, as are the tires (from touchdown and braking).
Here's a similar image of an F/A-18 performing a runup test prior to takeoff. This jet is longer and larger than the F6F (though if you've ever stood under a Hellcat you'll notice they're not exactly small planes), and as you can plainly see, the entire back half of the aircraft glows white-hot:

So clearly jets are more visible targets for an IR missile, though a prop plane wouldn't be that hard for an IR seeker to find.
The biggest issue relating to getting a good IR lock and track is contrast. Against a blue sky, almost any airplane is readily visible to the missile's seeker. The biggest problem for combat aircraft in early engagements was firing at a target below the horizon (so-called "look-down, shoot-down"). This was a problem for both radar and IR technologies. For IR, solar heating of the ground produced high enough temperatures that the aircraft could not be distinguished unless it had turned its tail to the shooting aircraft and was showing its 600-degree exhaust plume. Nothing on the ground underneath it would be that hot. Prop planes, while operating at lower temperatures, would still be warmer than the ground, and unlike a jet the aspect angles at which this heat is most noticeable is around the front half of the aircraft.

Bubble canopy -- F8F Bearcat, perhaps?  Not that this detail changes the point you're making.

Cameras these days automatically adjust focus and contrast. The white color of the first photo is not necessarily at the same temperature as the white color in another photo. On top of this, personally I find it impossible to believe that the wheel is as hot as the engine exhaust.

@DrZ214 the frequency of the light and intensity is usually transformed into the false color image you see.  In that the temperature of two regions having the same color does not necessarily indicate an identical temperature.
As for brake temperature, it isn't unusual for brake pads to exceed engine operating temperatures. Brakes are not usually cooled once the vehicle stops moving.


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Why do these tail-sitters not have bigger propellers in their main engine?
My attention was called to tail-sitter aircraft, like these two:

Convair XFV Pogo By No information - US Navy, Public Domain, Link

Lockheed XFV 1 Public Domain, Link
These aircraft land on their tails but transition to normal flight during cruise.
But what I want to know is, why are the propellers so small? Disk loading is a big part of efficiency. That's one reason why helicopter blades are so big. It is more efficient to accelerate a large mass of air to a slower velocity than a small mass of air to a faster velocity.
As far as I know, the disk loading factor is valid for propellers in horizontal cruising too.
So why aren't the propellers any bigger? They have plenty of room at the nose. No worries about ground clearance. I would think for a helicopter-like takeoff, this puts a lot more strain on the engine and thus efficiency would be even more important than usual.

@ymb1 Well both the Pogo and XFV have contra-rotating props which cancel out the gyro forces, at least for the propellers themselves.

They wanted to fly them fast, and efficiency in hover was less of a concern.

@PeterKämpf How would a larger propeller limit airspeed during cruise?

@DrZ214: Please read here. To fly fast, tip speed must be limited.

You're correct that it is more efficient to accelerate a large mass of air to a slower velocity. But then there are other things to consider here:

The attitude control i.e. transition is based on the downwash generated by the propellers (unlike helicopters). Either you should have high speed downwash or reach higher speeds for the flight controls to become operative. Reaching higher speeds in vertical flight would tax the engines further and the higher speed propellers, however less efficient would have been the better choice.
Huge rotors, like in helicopters would've been efficient, but also bring the related problems of blade flapping, lead and lag and droop, especially leading to problems during transition. There are also issues with tip speed, especially for the high speed interceptor role these aircraft were intended for.
One of the design considerations was shipboard (even submarines were talked about) use, conditions in which higher diameter rotors would've been a headache- you have to find ways to fold them, for one.

In the end, use of smaller rotors would've been influenced by a much more mundane reason- availability of powerful engine. Note that both of these aircraft were powered by variants of the same engine- modified Allison T40 turboprops, the only engine rated for vertical takeoffs and landings available- which consisted of two Allison T38s lashed together to run the same gearbox with contra-rotating propellers.
In fact, the heavier Lockheed XFV never made the transition flight, waiting for a higher powered T-40 and did only horizontal flights with long landing gears (which would've precluded using higher diameter rotors- though I'm not sure if that was a consideration).


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Do anti-collision lights in aircraft lose intensity over time?
Do anti-collision lights installed in aircraft lose their intensity over time and need to be replaced every few hundred hours of operation?

Wouldn't the manufacturers love that.

No, however the intensity is a feature of the bulb rather than the purpose.
Historically, incandescent bulbs were used, followed by Xenon flashtubes. Currently, LEDs are used in anti-collision lights and other lights on aircraft.
Please note that there are some requirements needed for bulbs used. They are mentioned in 14 CFR 25.1399:

Show a white light for at least 2 nautical miles at night under clear
  atmospheric conditions.

The Xenon flashtubes don't seem to lose intensity, but they do start to intermittently fail to fire as they age.  LEDs OTOH are well known (as a general technology) to lose intensity over time.


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How to measure the Aircraft cabin air quality?
Is there any tool or mechanism to do quantitative analysis of aircraft cabin/cockpit air quality?

A few years ago, the Australian Civil Aviation Safety Authority convened an  Expert Panel on Aircraft Air Quality.
From page 138 of the report they discuss the available monitoring devices. From my brief readthrough, there were no real monitoring devices for this purpose at the time (2009) however research was being conducted.

its specifically for the Bleed air contamination but if we have some other ways which have contaminated the Cabin air quality either CO, or if some passenger is able to get some liquid poisonous gas in the permissible limits of less than 100ml.